In general terms, soil exhibiting a hydric deficit with respect to the optimum water content must, in order to be able to be used in the construction of civil engineering works, in the embankment, the platform, the capping layer, the subbases or the pavement layers, be treated by a soil treatment method for improving and/or stabilising it.
Many natural soils contain clays in highly variable proportions. Clays represent a family of minerals that become unstable in the presence of water; the soil then goes from the solid state to the plastic state. Clays are thus responsible for instability in constructed structures.
“Soil treatment” means, within the meaning of the present invention, a method consisting of modifying the soil so that it fulfils the function for which it is intended, in particular improvement and stabilization of the soil. A soil treatment therefore does not include for example the insertion in the soil of an impermeable layer, since such an operation does not modify the soil per se.
“Improvement” of soils means the insensitization of the soils to water by flocculation of the clays, which allows travel, short term or even immediate, of site machinery and a modification in the long term, making the treated soils insensitive to water.
“Insensitivity” to water means that the soil no longer changes towards the plastic state, during the subsequent addition of water, either by natural capillarity or through rain or flood; the change of the soil to the plastic (ticky) state and a reduction of the bearing capacity of the soil, which causes the destruction of the structure, is thus avoided. The improvements concern particularly the embankment and the preparation of soil with a view to its stabilization.
“Stabilization” of the soils means increasing the mechanical performance for re-use in the upper layers of the structure; platform, capping layer, subbase and pavement layers. Stabilization can be obtained on materials that have undergone the improvement phase by the addition or hydraulic of pozzolanic binders or by increasing the proportion of lime, which causes a pozzolanic reaction with the compounds present in the soils or a syntaxic effect in the chalky soils.
The term “lime” represents a set of compounds which are quicklime, principally composed of calcium oxide, slaked or hydrated lime, principally composed of calcium hydroxide, or an aqueous suspension produced on the basis of quick lime or hydrated lime, for example milk of lime.
In order to stabilize and improve dry soils, the methods known at the present time are treatments that are conventionally carried out with quicklime or, in the cases of fine dry soils such as clayey silt, the treatment can be carried out with milk of lime.
For a dry soil treated with lime, the dry soil treatment method requires the following steps:
1. preparation of the soil, which consists of a breaking up and loosening of the compact soil;
2. pre-moistening of the soil, generally by spraying; this pre-moistening having two objectives:
                a) compensating for the hydric deficit of the soil in order to bring it to its optimum density;        b) adding the water necessary for the slaking of the lime and/or the development of the pozzolanic reactions;3. mixing of the soil in order to distribute the added water; in many cases this operation is performed on the day before the lime treatment;4. spreading the lime on the pre-moistened soil;5. mixing the soil with the lime;6. spreading water in order to adjust the content to the optimum for the type of soil treated;7. final mixing.        
When fine soils are involved, sensitive to water, for example clayey silts, the treatment can be carried out with milk of lime. This type of treatment is for example very widespread in the United States. The treatment then comprises the following steps:
1. preparation of the soil, which consists of a loosening of the compact soil;
2. pre-moistening of the soil, by spraying, in order to partly compensate for the hydric deficit;
3. mixing the soil, in order to distribute the added water;
4. spreading the milk of lime on the pre-moistened soil;
5. mixing the soil treated with milk of lime;
6. spreading any water, in order to adjust the content to the optimum for the type of soil treated;
7. final mixing.
Unfortunately these two treatment methods, respectively with lime or milk of lime, present many drawbacks, including a high number of travels of various items of machinery, which increases costs and the performance time and causes a high consumption of energy.
On sites where the soils are treated with quicklime, the quality of the mixing obtained is also disadvantaged by an overdosing of water during the pre-moistening. This overdosing causes the soil to go into a plastic state, which interferes with the travelling of earthworks machinery and causes the formation of lumps of lime, which are not distributed in the treated soil, when the soil is mixed. Moreover, the spreading of powdery lime may give rise to dust, in the event of excessively high wind, and require the work to be stopped.
The addition of water by spreading on the soil also causes trickling on sloping soils and in impressions left by the passage of the wheels of the spreading machinery, causing irregularity in the water content of the soil.
The spreading of milk of lime also leads to an overdosing of water during the pre-moistening in the points where there is an accumulation—in the machinery wheel ruts, at the bottom of slopes, etc. This phenomenon reduces the bearing capacity of the soil and interferes with the subsequent passage of site machinery. In addition, when the milk of lime is spread, the trickling on sloping soil or in machinery ruts also results in an irregularity in the lime contents in the soils treated.
In addition, the spreading is traditionally controlled by marking a surface over which the content of a tanker is to be spread. The difficulty in adjusting the flow rate to the speed of progress of the tanker frequently results in not achieving the target (overdosing) or achieving it before the tanker is empty (under dosing). This poor control of the flow spread for a given surface causes an uneven addition of lime and water on the soil.
There also exist, in the field of agriculture, water plowers provided with distributing teeth or plowing blades, also distributing, and which allow a plowing of loaded liquids such as liquid manures, in the furrows formed in agricultural soils. This equipment, for its part, has limits in use in several domains. In particular, they cannot be used on stony soils; when there are large blocks present (>150 to 200 mm), which create a resistance to the penetration of the teeth. A mechanism incorporated in the spreader then makes the teeth emerge from the soils and the liquid is situated on the surface with all of the problems related to trickling. Moreover, their plowing depth is limited to approximately 15 cm, and the addition of liquid is limited to 25 to 30 dm3/m2.
Finally, the transverse distribution of liquid is not regular over the entire width of the passage of the machines. Liquid is situated essentially in the direct vicinity of the teeth or blades. This unevenness will not be corrected during the following steps of the soil treatment, even during mixing. This is because the mixers work the soil in the direction of travel of the equipment rather than transversely.
As equipment of this type the one described in the British Patent Application GB-A-2180431 can be cited. In this document the soil treatment method comprises                digging in the soil, to a given depth, several parallel trenches simultaneously, and        distribution of a treatment liquid in the said trenches, possibly containing a matter in suspension.        
This equipment has the same drawbacks indicated previously.